This invention relates to a method for effectively conditioning uneven or irregular surfaces of industrial products made of a metal, plastic or the like for subsequent baking finish painting or force-drying painting.
In general, a metal blank having uneven surfaces has been painted in consecutive steps of (1) providing a product to be coated or painted, (2) degreasing, (3) treating chemically formed films, and drying, (4) priming coating, drying, and abrading, (5) selective puttying, drying, and abrading, (6) secondary puttying, drying, and abrading, (7) intercoating, drying, and abrading, (8) face or finish painting, and drying. (The drying is the force-drying at a temperature of the order of 80.degree. C. or baking at about 110.degree. C. to 150.degree. C.) In case of plastic products, the step (3) of treating chemically formed films and drying may be usually dispensed with, and the setting of coated films may be typically carried out at lower temperatures (80.degree. C. to 100.degree. C).
If parts of the blank surface wider than 5 mm.sup.2 are locally exposed as a result of the abrading step, these parts must be treated over again from the priming coating in the step (4) in order to obtain corrosion-resistance and adhesion. Particularly, magnesium-based blanks which may exhibit the high metallic activity must be treated over again from the treatment of chemically formed films in the step (3), or else it becomes very difficult to obtain the sufficient corrosion-resistance and adhesion.
More particularly, in case of die cast products which have in general pin holes, wrinkles, shrinkage cavities, cracks or the like in their surfaces, the steps (4) to (7) for conditioning the surfaces to be painted require the high cost of labor. Therefore, there has been a strong need for an improved precision die casting to obtain products having smoother surfaces. In the real practice, however, the smoothness of die cast products is of the order of 300 .mu.m at the best. Thus, it has been strongly required to lower the cost by improving the coating or painting processes.
The paints used in the priming coating are called as primer, undercoating paint, primer surfacer or the like and selected from kinds of paints suitable to respective blanks to be coated. Such selected paints are superior in corrosion-resistance and adhesion to blanks and to intercoating paints, and suitably diluted with respective exclusive thinners and applied to the blanks usually by means of spray coating.
If the paints used in the priming coating had a performance making smoother the uneven surfaces of blanks, the most expensive puttying processes could be dispensed with. In more detail, it will be best that the paints applied to blanks for priming coating in a usual manner expand in recesses by heating to provide smooth surfaces for the face or finish painting.